1. Field of the Invention
The present invention relates to a position detector assembly for a control rod in a nuclear reactor, and in particular relates to a control rod position detector assembly employing a magnetostrictive wire which works in cooperation with a control rod drive unit to continuously monitor the longitudinal position of the control rod relative to the reactor core.
2. Description of the Related Art
FIG. 8 is a cross-section of the construction of a pressurized water nuclear reactor. As shown in the figure, the output of the reactor is controlled by control rod drive units 5 disposed in an upper portion of a reactor vessel 6, which insert and extract control rods 4 into and out of the reactor core. The control rods are moved longitudinally by longitudinally moving drive shafts 3 connected to the control rods 4 within pressure housings 1. The positions of the drive shafts 3, that is, the positions of the control rods 4 relative to the reactor core, are detected by control rod position detector assemblies 7 comprising detector coils 2 disposed around the outer circumference of each of the pressure housings 1.
FIG. 9 is a cross-section showing the relationship between the control rod drive unit 5 and the conventional control rod position detector assembly 7. As shown in the figure, the conventional control rod position detector assembly 7 comprises detector coils 2 mounted on the outside of the pressure housing 1 of the control rod drive unit 5. Detector coils 2 corresponding in number to the length of the control rod 4 when withdrawn from the reactor core, usually forty-two, are mounted with even spacing on a coil support pipe 8 on the outside of and coaxial to the pressure housing 1. In anticipation of events such as breakages in the wiring, the detector coils are divided into two systems consisting of an A system comprising the set of alternate detector coils 2a and a B system comprising the set of detector coils 2b. The spacing between adjacent detector coils is approximately 90 mm, or when each system is considered separately, 180 mm because of the alternation. At the same time, the drive shaft 3 of the control rod drive unit 5, which is the portion whose position is detected, is usually composed of a stainless magnetic material. As a result, the drive shaft 3 itself is magnetized because the magnetic field from the control rod drive unit 5 is strong.
Since the temperature within the pressure housing 1 is approximately 300 degrees Celsius, limits on the working temperature of the insulating materials used in the detector coils 2 of the control rod position detector assembly 7 make cooling the coils compulsory. For that reason, air is supplied to the space 9 between the pressure housing 1 and the detector coil support pipe 8 on which the detector coils are mounted and the detector coils are air-cooled from within as shown in FIG. 9.
Next, the method of detecting the position of a control rod 4 by means of the detector coils 2 of the control rod position detector assembly 7 will be explained. When the magnetic drive shaft 3 passes through the center of a detector coil 2, an electric potential is induced in the detector coil 2 and as a result the impedance in the detector coil 2 changes. Consequently, by detecting the changes in impedance in each of the detector coils 2, the position of the tip of the magnetic drive shaft 3 can be detected as it moves inside the pressure housing 1 of the control rod drive unit 5, and thus the position of the control rod 4 within the nuclear reactor can be ascertained.
Also, in order to ensure the reliability of the reactor, it is necessary to measure the descent times (insertion times) of the control rods. The method of measuring the descent times of a control rod by means of the control rod position detector assembly 7 is to measure the insertion times from when a control rod starts to descend until it reaches a dashpot 10 (see FIG. 10) by means of the changes in electric potential (changes in velocity) in the generated electric currents which depend on the descent velocity of the magnetic drive shaft 3 as it passes through the detector coils 2. Thus, as shown in FIG. 11, when the descent velocity of the control rod 4 is fast, the electric potential of the electric current generated in an detector coils 2 rises, and when the velocity of the control rod 4 suddenly decreases, the electric potential suddenly decreases. In order to make use of such changes in the electric potential of the electric current generated in the detector coils 2, the dashpots in a fuel assembly which decelerate control rods 4 by means of fluid resistance are each disposed in a position approximately 85 percent of the fully inserted position. Consequently, the position of each control rod 4 can be precisely determined by a sudden decrease in electric potential at a position such as T1 shown in FIG. 11.
Moreover, whether or not the control rod 4 has been completely inserted into the reactor core is determined, as shown in FIG. 11, by detecting the rebound waveform R up to the rest point T2, that is, the waveform of the rebounding of the drive shaft 3 due to shock absorbing springs mounted on the control rod clusters as the drive shaft 3 of the control rod 4 reaches the bottom end.
However, since the position of the tip of the drive shaft 3 is detected by changes in impedance in the detector coils 2, signals indicating the position of the control rod 4 can only be obtained at the positions of the detector coils 2. In other words, the intervals at which the position of the control rod 4 can be detected, depend on the spacing at which the detector coils are mounted, which is approximately 90 mm. Generally, a control rod drive unit 5 drives a control rod 4 in steps, the length of each of these steps being approximately 16 mm. Consequently, one problem is that the physical position of the control rod 4 can only be confirmed at intervals corresponding to several drive steps.
Furthermore, the detector coils 2 are divided into two systems, the A system and the B system, and when one system cannot be used because of circuit failure, the position can only be detected at the single system intervals of 180 mm, in other words, intervals approximately ten times the length of a drive step of the control rod 4, further reducing accuracy. Consequently, from the viewpoint of a protective system for the reactor, there is a need to consider the uncertainty of the position of the control rods when designing reactor cores.
In addition, as explained above, measurement of the descent times of the control rods according to the control rod position detector assembly 7 involves measuring the insertion times from the commencement of descent until a dashpot is reached by means of the changes in electric potential in the electric currents which depend on the descent velocity of the magnetic drive shaft 3 as it passes through the detector coils 2, and it is well known that the descent times of a control rod cannot be accurately measured when the descent velocity is slow because the electric potential is low, making the commencement of descent, the position of the dashpot, and the fully inserted position unclear.
Moreover, in a rare event such as the control rod 4 stopping during descent, it is impossible to confirm the rest position of the control rod 4 (fully inserted or partway), and therefore the accuracy and reliability of the detection of the position of the control rod is low.
Similarly, when the rebound waveform used to determine whether the control rod 4 has been completely inserted is smaller than the descent velocity, the former is often unclear, and it is therefore difficult to detect whether the control rod 4 has been completely inserted into the reactor core.
In addition, due to limits on the working temperature of the insulating materials used in the detector coils 2 of the control rod position detector assembly 7, the ambient temperature around the detector coils 2 must be vigorously cooled and it is necessary to ensure that design conditions are not exceeded, requiring that the volume of the cooling equipment for the control rod drive unit be made quite large so that it can handle the large amounts of heat given off by the housing 1 which is heated to temperatures as high as about 300 degrees Celsius.